Impedance measurement systems can be utilized in a variety of applications, including in a radio transceiver. For example, the impedance (e.g., the transmission line impedance) between an antennae (e.g., the load) and a transceiver (e.g., the power source) is often measured and adjusted to ensure maximum power transfer between the transceiver and the antennae, as is well known in the art. Maximum power transfer is ensured by making the transmission line impedance equal to the output impedance of the load. An example of a system that adjusts the impedance of the transmission line in a radio transceiver is disclosed in U.S. Pat. No. 4,311,972, entitled "High Speed Antennae Coupler." The coupler measures power transfer parameters at two different points along the transmission line and adjusts the impedance of the transmission line in accordance with the power transfer parameters.
The impedance of the transmission line and the antennae is not a constant impedance. Many variables can drastically affect the impedance of the transmission line and the load (e.g., particularly at high frequencies associated with radio transmissions). Examples of these variables are the frequency of the signal that is being applied to both the transmission line and the load, environmental conditions, weather conditions, manufacturing tolerance of the transmission line and the load, position of the antennae, altitude of the antennae, and combinations of the enumerated conditions. These enumerated conditions can drastically affect the operation of the transceiver and of the power transfer between the power source and the load which is normally an antennae.
Typically, a tunable impedance network between the power source and the load is adjusted so the output impedance of the power source matches the output impedance of the transmission line and the load. Conventionally, the load or the antennae has an impedance of 50 ohms under ideal conditions. The impedance network can be adjusted by a control circuit which determines a factor related to impedance based upon measured signals associated with the transmission line. For example, U.S. Pat. No. 4,506,209 discloses an impedance measurement system that utilizes a first discriminator which generates three analog signals that represent the reflected voltage, the forward voltages, and the phase difference between the reflected voltage and the forward voltage. The first discriminator receives an injection signal from a second discriminator. The injection signal has a frequency offset from the frequency of the power source. The analog signals are converted by a digital-to-analog converter and are provided to a computer to calculate the impedance between an antennae and a transceiver.
Heretofore, impedance measurement systems have been expensive, somewhat inaccurate, slow, and noisy. Some conventional systems require a large number of samples to accurately calculate impedance. These systems take longer to measure impedance. Others require significant circuitry to sense parameters from which the impedance is calculated. Still, other systems rely on time-consuming iterative techniques to determine impedance. Also, some conventional systems can cause noise to be injected or to be reflected back onto the transmission line.
Thus, there is a need for an impedance measurement system which is faster, more transparent, and less costly. Further still, there is a need for an impedance measurement system which can determine impedance within one cycle of a sinusoidal signal.